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Astronomers find universe’s dimmest known galaxy

By Rachel Courtland

The most dark matter-dominated galaxy in the universe, Segue 1, is shown among background and foreground stars in a Sloan Digital Sky Survey image (left). The galaxy’s 24 known stars have been singled out in the image at right (Left image: SDSS; right image: M Geha)

Astronomers have identified what appears to be the dimmest galaxy in the universe. Although it shines with the brightness of only a few hundred Suns, it seems to be full of dark matter, making it an ideal candidate to search for evidence of the mysterious material, they say.

The galaxy, known as Segue 1, is one of roughly two dozen dwarf galaxies that orbit the Milky Way. It sits close to the Sagittarius stream, a river of stellar debris torn from another dwarf galaxy.

So far, only 24 stars are confirmed members of Segue 1. Because it is so dim, astronomers originally thought it was a tight-knit group of stars called a globular cluster.

But follow-up observations by Marla Geha of Yale University and colleagues suggest it is indeed a dwarf galaxy. They measured the velocity of its stars and found they were all moving at roughly 5 kilometres per second, 10 times faster than expected if they were gravitationally bound only by each other.

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The stars must be surrounded by a lot of matter to prevent them from flying off at such speeds. Although the total brightness of the galaxy adds up to less that 350 times the brightness of the Sun, it must weigh at least 450,000 Suns – and possibly 20 times that amount if it is as large as other dwarf galaxies.

There is little gas in the system, suggesting almost all of that mass seems to be made up of dark matter, the mysterious stuff that makes up most of the universe’s mass.

“It is the most dark matter-dominated galaxy in the universe,” Geha told New Scientist.

Dark matter search

The object is one of a larger group of recently discovered galaxies where dark matter seems to outweigh ordinary matter by a factor of 100 or 1000. In the Milky Way, dark matter is thought to outweigh luminous matter by a factor of 10.

“These galaxies are basically invisible,” says team member Beth Willman of Haverford College in Pennsylvania. “We didn’t know these kinds of galaxies existed until a couple of years ago.” That’s when large surveys, like the Sloan Digital Sky Survey, began turning up these galaxies, by detecting slight clumps – or “overdensities” – of stars in the sky.

Because Segue 1 contains such a high concentration of dark matter, it might be an ideal place to look for evidence of dark matter annihilation.